A series of poly(butylene terephthalate) copolyesters containing 5-tert-butyl isophthalate units up to 50 mol %, as well as the homopolyester entirely made of these units, were prepared by polycondensation from a melt. The microstructure of the copolymers was determined by NMR to be random for the whole range of compositions. The effect exerted by the 5-tert-butyl isophthalate units on thermal, tensile, and gas transport properties was evaluated. Both the melting temperature (T-m) and crystallinity were found to decrease steadily with copolymerization, whereas the glass-transition temperature (T-g) increased and the polyesters became more brittle. Permeability and solubility slightly increased with the content in substituted isophthalic units, whereas the diffusion coefficient remained practically constant. For the homopolyester poly(5-tert-butyl isophthalate), all these properties were found to deviate significantly from the general trend displayed by copolyesters, suggesting that a different structure in the solid state is likely adopted in this case. (C) 2004 Wiley Periodicals, Inc.